Mammary epithelial cell fate and neoplastic transformation are regulated by the wnt/beta-catenin signal transduction pathways. Wnt genes are a family of growth factors whose signaling influence responses including, but not limited to: cellular development, differentiation and stem cell maintenance. In transgenic mice, expression of WntlOb under the regulation of MMTV-LTR (Wnt1 ObTG) results in precocious ductal formation and in the development of stochastic adenocarcinomas in the mammary glands in both male and female mice. Expression profiling using microarray analysis of Wnt10bTG -driven tumors reveals enhanced expression of mammary epithelial stem cell markers Krt2-6 CD49f and CD44 leading to the hypothesis that Wnt10b drives tumorigenesis by altering the mammary stem cell compartment/niche. The aims of this proposal are to isolate mammary epithelial stem cell(s) and to characterize the molecular profiles of the cells that drive Wnt10b-tumorgenesis, identifying the cells responsible for Wnt-mediated breast tumorgenesis. A novel transgenic mouse MMTV-Wnt10bTG-IRES-LacZ (Wnt1 ObTG-LacZ) will be used for the expression of LacZ to identify Wnt1 Ob-expressing cells. Upon verification of Wnt1 Ob-expression, FACS analysis and cell sorting to identify the expression levels of breast epithelial stem cell markers, including CD24, CD29, CD49f and Seal. Controls will be genetically ablated Wntl0b mice in comparison with wild type mice to determine the expression of the previously mentioned markers by FACS analysis. Isolated cells from Wnt1 ObTG-LacZ can then be use for serial-diluted xenographs experiments into SCID-Nude and or syngeneic mice to generate tumors. Conduct mammosphere assays and terminal differentiation assays to assess the reconstitution of myoepithelial, luminal and basal epithelial cells in vitro. These cells will be analyzed by immunohistochemistry for terminally differentiated markers. Conduct ChIP on chip assays on wnt-response stem cell markers with RNA Pol II beta-catenin and Lef-1 specific antibodies.